A universal, programmable active filter on a single silicon chip is an active filter which, by switchable programming through switches or with controllers or a microcomputer, or by application of control voltages to appropriate pins of the integrated circuit. The control signals are decoded on chip and applied to MOS devices operated as switches. This programming can be used to change the filter from one type of output such as a bandpass, to another type, such as allpass, lowpass, highpass or notch. Not only may the type of output of the filter be changed by software, but the filter's selectivity "Q", its gain, its center frequency (or cutoff frequency), and other parameters also may be changed. By the proper application of o and off signals to the gates of these MOS devices, capacitors and/or resistors may be switched in and out of the filter circuit, all as well known in the art. Therefore, resistor or capacitor values (or the parameters of a resistor/capacitor pair) may be varied using a microprocessor program which limits control pulses to these MOS gates.
Universal, programmable active filters are well known. Most employ an operational amplifier. Since the potential difference between the two input nodes of an operational amplifier must be close to zero volts, some form of resistive feedback loop is necessary from the output to the negative input. A parallel coupled resistor can serve this purpose. It allows the flow of d.c. current from the inverting input of the operational amplifier to the output. However, resistors are difficult to integrate on a single silicon chip in a manner where they can be programmed into and out of the circuit using MOS switches. Once the resistors are formed on the circuit, they have a fixed resistance. Switching resistors in and out using MOS switches causes distortions and inaccuracies in resistance values because the MOS devices themselves introduce unwanted cumulative resistance values which are difficult to control. To avoid this problem, prior art filters, for example the commercially available MF-10 programmable filter, uses external resistors rather than on chip resistors.
However, it is highly preferable for ease of use to have all necessary components directly on the chip. One way for providing the necessary feedback entirely on the chip without resistors would be to substitute capacitors which are easier to fabricate in a small area in an integrated circuit design. Capacitors can accurately be switched in and out of the circuit using MOS switches. However, using a parallel-coupled capacitor instead of a resistor does not permit d.c. current to flow between the input and the output terminals of the amplifier. This prevents the inverting input node from periodically being brought back to the d.c. offset voltage of the operational amplifier, as required.
One compromise technique which has been used is the provision of an additional large resistor in parallel with the capacitor coupled across the amplifier to permit d.c. current to flow to the inverting node. This allows a d.c. bias to be established at the inverting input node of the operational amplifier, placing a d.c. potential at that node. However, the integration of this circuit into a monolithic integrated circuit requires a large resistor and hence requires significant silicon real estate. Further, the value of such a large resistor is difficult to control repeatably.
The alternative is to use a switched capacitor instead of a resistor in parallel with the fixed capacitor. This technique has been described in an article entitled "An Electrically Programmable Switched Capacitor Filter" by David J. Allstot, et al., IEEE J. Solid State Circuits, Vol. SC-14, pp. 1034-1041, December, 1979. FIG. 2 of the Allstot, et al. article shows a circuit using a parallel connected switched capacitor to establish a d.c. bias voltage at the inverting input node.
The problem with these prior art circuits is that they can only be used to implement bandpass filters. They do not operate satisfactorily in the implementation of a lowpass, highpass, allpass or notch filters. When designing universal active filters to be integrated on a single silicon chip, it is far preferable to have a single design which can operate, selectively, as a lowpass, highpass, bandpass, allpass or notch filter. The filter of this invention is just such a universal filter. Furthermore, the use of the above prior art technique results in the degradation of Q and center frequency accuracy, a problem not encountered in the filters of this invention.
The technique employed by this invention to overcome the above difficulties is the use of control circuitry which periodically, under clock control, brings the inverting node of the operational amplifier back to a voltage approaching zero without need of any large external or integrated resistors. The circuit of this invention has the advantage of being able to operate at very low operating clock frequencies, such as 1 hz, while providing a high degree of d.c. gain stability over a large temperature range. The d.c. offsets at all nodes are satisfactory. The programmable universal filters of this invention can be reconfigured by a microprocessor to serve a wide variety of needs such lowpass, bandpass, allpass and notch filters.